tox: For XBlock, always use latest patch release within the minor release
tox: For XBlock, always use latest patch release within the minor release

Rather than using a specific patch release, always use whatever the
latest patch release is within the minor that we want to test.

hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock